An attractive strategy for cancer treatment is to strengthen the ability of the patient's own immune system to detect and clear cancerous cells. However, current antibody-based or cell-based immunotherapies have limited effectiveness or high invasiveness. A small molecule drug that activates the immune system to fight cancer would be an attractive clinical option because it may not require the intravenous dosing associated with antibody therapies and it would be less invasive than cell based options. The objective of our research is to develop a drug that stimulates the immune response to cancer by activating cells known as the gamma delta T cells. Gamma delta T cells are early responders to malignancy, and as such, their activation holds great promise for cancer immunotherapy. In contrast to T cells that express the more prevalent alpha beta T cell receptor and respond to peptide antigens, T cells that express the V?9V?2 T cell receptor respond to small phosphorous-containing compounds known as phosphoantigens. As a side effect of treatment for osteoporosis, nitrogenous bisphosphonate drugs indirectly activate gamma delta T cells by enhancing accumulation of the phosphoantigen isopentenyl diphosphate, substantially reducing the risk of some cancers. However, no known direct activators of gamma delta T cells are available for human use. Here, we propose to test the central hypothesis that phosphonate phosphoantigens with enhanced metabolic stability will directly activate gamma delta T cells, in a way that is dependent upon cellular internalization and integrin- mediated cell contact. Development of a new class of phosphoantigens is essential because clinical use of bisphosphonate drugs to activate gamma delta T cells is limited by their narrow therapeutic index and high bone affinity, and clinical use of natural phosphoantigens is limited due to rapid diphosphate metabolism. We will synthesize a series of novel organophosphorous compounds with increased metabolic stability relative to known phosphoantigens and characterize their activation of gamma delta T cells. We also will prepare protected phosphoantigens, which will allow us to continue to investigate how enhanced cellular uptake promotes a stronger response. We will use these new chemical tools to address the fundamental unanswered question of how phosphoantigens activate gamma delta T cells to fight cancer. The ultimate goal is to identify a phosphoantigen that will be used for cancer treatment. These findings will come at a time when the biological understanding of anti- cancer immunity is far from complete, and thus have the potential for dramatic impact on the field of cancer immunotherapy.